1. Field of the Disclosure
This invention pertains to a method for making a printing form having a relief surface, and in particular to a method for making a printing form from a photopolymerizable printing element.
2. Description of Related Art
Flexographic printing forms, such as printing plates, plates-on-sleeves, and printing cylinders, are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing forms are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing forms can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing forms are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically, the printing form is uniformly exposed through its backside, i.e., backflashed, to a specified amount of actinic radiation to form a floor. Next, the form is imagewise exposed through its front side with the same actinic radiation that was used for the backflash exposure. The imagewise exposure is made through an image-bearing art-work or a template, such as a photographic negative or transparency (e.g. silver halide films), or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer. The actinic radiation exposures can be conducted with ultraviolet (UV) or black light. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden and/or become insoluble to solvents used during development. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or in-situ mask during exposure do not hardened and/or remain soluble. The unexposed areas are removed by treating with washout solutions or heat leaving a relief image suitable for printing. If treated with washout solutions, the form is dried. The printing form can be further exposed to complete polymerization, i.e. post-exposure, and to remove surface tackiness, i.e., finishing exposure. After all desired processing steps, the form is then mounted on a cylinder and used for printing.
The three exposure steps, i.e., backflash, imagewise exposure, and post-exposure, can be accomplished using actinic radiation from a variety of actinic radiation sources, such as for example, lamps. Most often the printing form is exposed to a bank of lamps in an exposure unit based upon a pre-determined length of time, i.e., exposure interval. Exposure times vary from a few seconds to a few minutes depending upon the output of the lamps, distance from the lamps, desired relief depth, and the thickness of the plate. The predetermined exposure time can be derived from a test on a representative sample of the printing form at the same or similar conditions used ordinarily to prepare the printing form. The test involves exposing the sample to a series of different times, treating the sample to remove unpolymerized material, and analyzing the resulting sample. The test may be referred to as a backflash test or a main exposure test. In the backflash test, the sample is exposed through the support, and the resulting sample is measured for thickness of the floor. The time providing the desired floor thickness is the pre-determined time for the back exposure. In the main exposure test, the sample is exposed through a test target image (i.e., phototool or in-situ mask layer), and the resulting sample is analyzed for relief image quality, e.g., relief structure holds fine lines and highlight dot areas, and adequate depth in reverse images. The time providing the desired relief image is the pre-determined time for the main exposure.
However it may be necessary to frequently recalibrate the predetermined exposure time/s for consistent results. Time of exposure can be influenced by the lamp intensity, spectral energy distribution of the radiation emitted from the source, the distance from the photosensitive element, the desired image resolution, and the nature and amount of the photopolymerizable composition in the element. Lamp intensity diminishes with use, and the predetermined exposure time that was used to prepare the printing form becomes inadequate to cure or crosslink subsequent printing forms. Even when the lamp or lamps are replaced, the light intensity drops off in the first 20 hours of lamp life, so that recalibration is necessary throughout this initial age-in of the lamps. Factors which affect the replacement of lamps are the physical location of the lamps within the hood, their elapsed operation time, and the elapsed operation time of all adjacent lamps. Frequent recalibration is an is undesired step that can consume considerable platemaking time and manpower, as well as printing forms.
Exposure units are known to have a radiation integration system, sometimes referred to as an integrator, which evaluates the intensity of the lamps illuminating the bed where the printing form lies. An example of an exposure unit having an integrator is the CYREL®4260ECLF. The integration system compensates the time of exposure according to the intensity of the radiation emitted by the lamps. The system may include a photocell that senses the radiation incident thereon, and a circuit that integrates a signal from the photocell. The photocell in these exposure units typically measures the intensity of the lamps for a broad spectrum of wavelengths of the emitted radiation. The circuit integrates the photocell signal until a predetermined circuit output signal is achieved which corresponds to a particular exposure value. Such exposure control systems, should in theory provide the designed for exposure values. In practice however, since exposure is a function of many variable factors, there is potential for the exposure values actually produced by any such system to vary from the values designed for. These variations in exposure values often result from the cumulative effect of variations, typically within prescribed tolerances, of the optical, electronic, etc. systems.
Thus, it is desirable to insure proper exposure of photosensitive elements consistently over the useful life of the lamp/s in an exposure unit. It is also desirable to avoid the time, manpower, and materials associated with recalibration of lamps to determine appropriate time interval of exposure for photosensitive elements. It is also desirable to insure proper exposure of photosensitive elements necessary to achieve satisfactory resulting relief structures for printing forms